Method of producing aggregated kaolinite pigment using organic silicon compounds

ABSTRACT

An improved process for producing a aggregated kaolin pigment having desired physical and optical characteristics when used as a coating, filler or pigment in paper. The aggregated pigment is produced by contacting a substantially dry fine particle size kaolin with a previously hydrolyzed organic silicon compound, such as hydrolyzed tetraethoxysilane, and then recovering the aggregated kaolin pigment.

RELATED APPLICATION

This application is a continuation-in-part of our commonly assignedcopending application Ser. No. 07/877,541, filed May 1, 1992now U.S.Pat. No. 5,232,495.

BACKGROUND OF THE INVENTION

This invention relates to the preparation of chemically aggregatedkaolin pigments, using organo-silicon compounds. The products are usefulas fillers and coatings for paper.

Kaolinite occurs naturally as clay, which is mined and processed toproduce kaolin pigments for use in paper filling and paper coatingapplications. In general, the objectives of using a kaolin pigment areto improve qualities of the paper product, such as opacity, brightness,smoothness, printing, porosity, surface coverage, light scatter, and toreduce the cost of paper manufacturing. Various methods are commonlyemployed to enhance the performance of a given kaolin pigment. Amongthese are calcining and chemical aggregating methods which improve thebrightness and opacity imparted to paper by a given kaolin pigment.

The brightness and opacity imparted to paper by a given kaolin fillerare quantitatively related to a property of the filler identified as the"scattering coefficient S." This scattering coefficient S of a givenfiller pigment is a property well known and extensively utilized in thepaper technology art, and has been the subject of numerous technicalpapers. The early exposition of such measurements was made by Kubelkaand Munk, and is reported in Z. Tech Physik 12:539 (1931). Furthercitations to the applicable measurement techniques and detaileddefinitions of the said scattering coefficient are set forth at numerousplaces in the patent and technical literature. Reference may usefully behad in this connection, e.g., to U.S. Pat. Nos. 4,026,726 and 4,028,173.In addition to the citations set forth in these patents, reference mayfurther be had to Pulp and Paper Science Technology, Vol. 2, "Paper,"Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, N.Y.).

In a filled paper, higher light scattering allows paper to look moreopaque without increasing light absorption. The use of pigment with ahigher light scattering coefficient allows reduction in either the basisweight or amount of filler required to achieve targeted properties; forexample, opacity and brightness. Traditionally, this has been achievedusing titanium dioxide, calcined clays and precipitated calciumcarbonate. The relatively higher light scattering of titanium dioxide isdue to higher refractive index. Higher light scatter observed withcalcined kaolin and precipitated calcium carbonate is believed to be dueto the intrinsic porous structure developed during the process ofmanufacturing of these pigments. See McConnell et al, U.S. Pat. No.4,381,948.

In general, the attempt to increase light scatter by modification ofkaolinite mineral also induces some increase in pore void volume. Inaddition, such modification can produce pigments with particle sizedistribution in a fairly narrow range. For example, calcining of finekaolinite above its dehydroxylation point can produce a product withincreased pore void volume. In U.S. Pat. No. 4,826,536, issued May 2,1989, to Raythatha et al., similar aggregation is achieved chemically byreacting fine kaolinite clay with rapidly hydrolyzing metal chlorides.The acidic byproduct of this reaction may be neutralized with gaseousammonia. In Raythatha et al., U.S. Pat. No. 4,818,294, and in Brown etal., U.S. Pat. No. 4,935,062, similar aggregates may be produced byreaction with organo-silicon compounds that are non-corrosive andproduce no solid byproducts. The light scattering coefficient and porevoid volumes of these clays are significantly higher than the startingkaolinite material.

Marginal increase in light scatter (generally less than 10 units) may beinduced by mixing kaolinite particles of different size or by chemicalflocculation. However, these structures are generally unstable and wouldbreak down under the high shear stress of paper making or paper coating.

Aside from use as fillers, the aggregated pigments are used in papercoating to improve surface coverage. The application of such pigmentscan lead to a smoother surface, higher porosity, gloss and printproperties. In Raythatha U.S. Pat. No. 4,818,294 a chemically aggregatedkaolin pigment is shown to significantly increase coated sheetproperties, especially paper and print gloss.

In more detail, in U.S. Pat. No. 4,381,948 to McConnell et al., acalcined kaolin pigment and a method for manufacture of same aredisclosed. The said pigment consists of porous aggregates of kaolinplatelets, and exhibits exceptionally high light scatteringcharacteristics when incorporated as a filler in paper. This pigment,which substantially corresponds to the commercially available productALPHATEX® of the present assignee, ECC International Inc. (Atlanta,Ga.), is prepared by first blunging and dispersing an appropriate crudekaolin to form an aqueous dispersion of same. The blunged and dispersedaqueous slurry is subjected to a particle size separation from whichthere is recovered a slurry of the clay, which includes a very fineparticle size; e.g. substantially all particles can be smaller than 1micrometer E.S.D. The slurry is dried to produce a relativelymoisture-free clay, which is then thoroughly pulverized to break upagglomerates. This material is then used as a feed to a calciner; suchfeed is calcined under carefully controlled conditions to typicaltemperatures of at least 900° C. The resulting product is cooled andpulverized to provide a pigment of the porous high light scatteringaggregates of kaolin platelets as described.

Calcined kaolin products, including those of the aforementionedALPHATEX® type, are seen to be manufactured by relatively complextechniques involving a multiplicity of steps, including specifically acalcining step, plus various preparatory steps and post-calcining steps.Thus, the said product is relatively expensive to produce, and requiresconsiderable investment in complex apparatus and the like; e.g. highlyregulated calciners, etc. The particle size in the feed to the calcinermust be carefully controlled, because a relatively small increase incoarseness of such feed can have very marked detrimental effect onValley abrasion. Furthermore, calcination per se will produce anabrasive product if overheating occurs. Consequently, the conditions ofpreparation of calcined materials must be very carefully controlled inorder to keep abrasion acceptably low in the calcined product.

In Raythatha U.S. Pat. No. 4,826,536, a process is disclosed in which afine particle size kaolin is reacted in particulate form with a metalchloride, such as silicon tetrachloride, to form a chemically aggregatedstructured kaolin pigment. The metal chloride may be one or more of thechlorides having the general formula MCl_(x), where M is Si, Ti or Al;and X is 3 or 4 depending on the valence of M. Heating may optionally beused to shorten the reaction time. When so used, temperatures generallywill not, however, exceed about 150° C. In order to complete thepolymerization and condensation which is thought to occur, it ispreferable to age the resulting product for a period, typically at leastthree days. In another aspect of that process, additional improvementsin the products are found to occur by the addition of ammonia to thecombined kaolin and metal chloride.

Thus Raythatha U.S. Pat. No. 4,826,536 describes methods of preparingchemically aggregated kaolinite pigment using very reactive metalchlorides such as silicon tetrachloride and titanium tetrachloride. Theapplication of such reactive metal chloride leads to aggregated productsthat give enhanced optical and printability properties for filled andcoated papers. However, due to the nature of the aggregating chemical,very specialized equipment and processing steps are required. Inaddition, the resultant by-products are difficult to remove by ordinarymethods. Furthermore, the aggregated kaolinite pigment product cannot bedispersed effectively in water at solids contents above 50 weightpercent, which is an impediment to their shipping and use.

In Raythatha U.S. Pat. No. 4,826,536, free moisture present in theparticulate feed is useful to initiate at least a partial hydrolysis ofthe metal chloride or chlorides. The patentee indicates that if themoisture level is too high, however, it can diminish or impair theefficiency of the aggregation process. Thus in the instances wheresilicon and/or titanium chlorides are used, if the moisture level is toohigh relative to the metal chloride(s), then the hydrolysis productsfrom the metal chlorides will predominantly precipitate as the metaloxide, i.e. as silica gel and/or titania. In the case of aluminumtrichloride, if the moisture level is too high relative to the metalchloride, then the dilution of the finally resulting aluminum hydroxide(in the presence of ammonia) can be so high as to cause inefficientaggregation.

Raythatha U.S. Pat. No.4,818,294 discloses a method of producingaggregated kaolin pigments using organo-silicon compounds either singlyor in combination. In general, the compounds may be selected fromsymmetric compounds having the formula ##STR1## where R=CH₃, C₂ H₅, C₃H₇, n--C₄ H₉ sec-C₄ H₉ and C₆ H₅ and asymmetric compounds having theformula ##STR2## where R=C₂ H₅, X=Cl or Br, and R'=H.

In a typical process, substantially dry kaolin mineral is treated withan effective amount of an organo-silicon compound such as silanes havingthe formula (RO)₄ Si, where R is a lower alkyl group of 1 to 4 carbonatoms; e.g., methyl (CH₃) or ethyl (CH₃ CH₂). The R groups in the silanecan be the same or different. The resulting products exhibit increasedlight scatter, improved wet void volume and bulk, and have a structurethat is strong enough to withstand the high shear forces of paper makingand paper coating. The amount of organic silicon compound may range from0.1 to 3.0%, preferably from 0.2 to 2.0% by weight of dry kaolin.

Raythatha U.S. Pat. No. 4,818,294 also teaches that aggregationenhancing chemicals, comprising alkaline earth metal salts and lithiumchloride, may optionally be used, in amounts in the range of 0.05 to3.0%, typically 0.05 to 2% by weight of the salt based on the weight ofthe dry kaolin.

Raythatha U.S. Pat. No. 4,818,294 further teaches that feed moisture istypically in the range of 1 to 2%, preferably 1.0 to 1.75%, by weight ofthe feed clay. The most preferred range is 1.5 to 1.75 % by weight ofthe clay. Brown U.S. Pat. No. 4,935,062, a continuation-in-part ofRaythatha U.S. Pat. No. 4,818,294, discloses that the organo-siliconcompounds of Raythatha U.S. Pat. No. 4,818,294 are useful for producingaggregated kaolin pigments by the method of the Raythatha U.S. Pat. No.4,818,294 when the moisture content of the feed is increased into therange of 2.0 to 5.0% by weight of the clay. Brown further teaches thatmoisture content above 5% causes the clay to become sticky and difficultto handle.

Thus Raythatha U.S. Pat. No. 4,826,536, Raythatha U.S. Pat. No.4,818,294 and Brown U.S. Pat. No. 4,935,062 all disclose and teach thataggregated kaolin pigments can be produced by treating a kaolin claywith organo-silane or metal chloride compounds in the presence of aquantity of water that is sufficient to hydrolyze and polymerize theorgano-silane or metal chloride compounds, i.e. up to about 5% of theweight of clay. These patents teach that it is necessary to restrict theamount of moisture used, and Raythatha U.S. Pat. No. 4,826,536 teachesthat when higher amounts of moisture are present the hydrolysis productsformed are ineffective for producing aggregated pigment.

Using the prior art of Brown 4,935,062, one would process the crude clayto a dried product, and then mix in the selected organo-silane toproduce an aggregated product. In practice this procedure has been foundto require an aging period of at least three days from the time the clayis treated until it can be used--e.g. in paper coating. This hasresulted in severe impediments to ease of production and use of thepigments.

Additional prior art pertinent to the present invention includes:

U.S. Pat. No. 3,567,680, issued to Joseph Iannicelli disclosing thatmercaptopropyl silanes having the formula: ##STR3## wherein Z isselected from the group consisting of hydrogen, cation, alkyl, aryl,alkylaryl, arylalkyl and derivatives thereof; X is selected from thegroup consisting of alkyl, alkylaryl, and arylalkyl; and R1, R2 and R3are selected from the group consisting of hydrogen, cation and alkyl,are suitable for modifying kaolin clays to enable them to be used asreinforcing fillers for elastomers. It may be noted that in Iannicelli,only the trialkoxy mercaptopropyl silanes are considered. Blends ofthese mercapto organosilanes with amino organosilanes are alsodisclosed.

U.S. Pat. No. 3,364,059 to Marzocchi, discloses a method for treatingglass fibers to improve their bonding relationship to rubbers bytreating them with a silane containing a thio group.

U.S. Pat. No. 3,834,924 to Thomas G. Grillo, an amino organosilane isadded to a high solids content pigment dispersion or slurry to changethe slurry form into a thick, flocculated and plastic type that issuitable for extrusion and drying. Because a thick, cake-like product isformed, the amino organosilane and pigment dispersion are preferablymixed or blended directly in a solids mixing apparatus such as anextruder, designed to extrude the plastic mass in the form of acompacted rod type body which may be fed directly into a drier. Theproducts are useful as a filler for polyurethanes. As can be seen, thedescribed treatment is for the different purpose of forming aflocculated, plastic mass of the kaolin, not for the purpose ofaggregating fine kaolin particles to form aggregated fine kaolinparticles.

In U.S. Pat. No. 3,894,882 to Robert B. Takewell et al., a rotatingpelletizing drum is used to form pellets from clay such as kaolin clay.To avoid the problem of dust, a wetting liquid is introduced into thedrum, preferably steam or steam/water. The steam adds heat to thepellets to aid in drying them. There is an incidental mention of using"other suitable wetting liquids", an extensive list being given whichincludes silanes.

In accordance with the foregoing, an object of the present invention isto provide an improved process for producing an aggregated kaolinpigment product which possesses improved pigment bulk, porosity andlight scattering characteristics, and hence is useful as a bulkingpigment for coating of paper and paper board, and which may also be usedas an opacifier and light scattering filler for paper and paper board aswell as in other paper manufacturing applications.

Another object is to provide a method which permits use of a simplerprocess with fewer process steps and faster processing than presentcalcining and chemical aggregation methods, by eliminating the necessityfor aging, dry milling or calcining required by the present methods.

A further object of this invention is to provide a method for producinga chemically aggregated kaolin pigment by means of a process whichpermits handling and treatment of the kaolin feed in a substantially drystate, and which yet produces aggregates having sufficient strength toenable their effective use in coating applications.

SUMMARY OF THE INVENTION

Now in accordance with the present invention, a method is provided forproducing a kaolin pigment having enhanced optical and printabilityproperties when used in paper manufacture. Pursuant to the invention asubstantially dry fine particle size kaolin is contacted with a solutionof an aggregating agent comprising a previously hydrolyzedorgano-silicon compound selected from the group consisting of symmetriccompounds having the formula ##STR4## where R=CH₃, C₂ H₅, C₃ H₇, n--C₄H₉ sec-C₄ H₉ and C₆ H₅ and asymmetric compounds having the formula##STR5## where R=C₂ H₅, X=Cl or Br, and R'=H; and an aggregatedparticulate kaolin pigment is thereupon recovered as product.

The solution of aggregating agent and the kaolin are preferablycontacted with one another by being commonly dispersed in a gasphase--e.g. in air. Preferably the kaolin feed is provided as an airsuspension, and the aggregating solution as an aerosol is intermixedwith the kaolin suspension, e.g. in a turbulent mixing device such as aTurbulizer®. The aggregating solution can also be sprayed as an aerosolinto a suspension of the kaolin as the latter passes through a duct, oras the latter falls in curtain fashion from the end of a continuous loopconveyor feed belt.

Preferred aggregating agents for use in the invention includetetramethoxy silane and tetraethoxy silane, each of which has beenpreviously hydrolyzed.

The mixture of kaolin and organosilane preferably also includes anaggregation-enhancing agent selected from the group consisting of thealkaline earth metal salts. Preferable salts for such use are those ofthe divalent alkaline earth metals--calcium carbonate is especiallypreferred. From about 0.5 to 5% by weight of dry kaolin of theaggregation enhancing agent is usefully added, with 2-4% beingpreferred, and about 3% being typical.

In a typical process, a substantially dry fine particle size kaolinpigment (preferably including the aggregation-enhancing agent) istreated with an effective amount of a compound prepared by hydrolyzingan organosilane. Preferably at least 98% by weight of the feed particlesare of less than 2 μm E.S.D. and at least 96% are less than 1 μm E.S.D.Typically from about 0.5 to 3% hydrolyzed organosilane, is mixed withthe kaolin, based on the dry weight of kaolin, with the treatingsolution of the aggregating agent typically containing about 60% byweight of the organosilane. The products are recovered directly from themixing, e.g. from the Turbulizer®. Where tetraethoxy silane ("TEOS") isused, a still more preferred range is from about 2.2 to 3.0% hydrolizedTEOS based on the weight of the dry clay. The resulting products whenused in paper making, exhibit increased light scatter, improved wet voidvolume and bulk. The aggregates are found to have high structuralintegrity, i.e. they are strong enough to withstand the high shearforces of paper making and paper coating.

The organic silicon compound is hydrolyzed by mixing the compound withwater in approximately a molar ratio to the silicon compound, preferablytogether with an alcohol having solvating properties for both the waterand the silicon compound, and a catalytically active quantity of an acidor base catalyst for the hydrolysis. Although the hydrolysis can proceedin the absence of the alcohol, it is preferred to have same present inorder to facilitate the reaction by completion within a commerciallydesirable brief duration. In the presence of the solvating alcohol,about 2 to 3 minutes is typically required to complete hydrolysis. Thealcohol can comprise among others, ethanol, methanol or isopropanol,with ethanol being particularly preferred because of its availabilityand relatively low toxicity. It is important to utilize the hydrolyzedorganic silicon compound in a freshly prepared condition, as the nascentsilicic acid which is formed during the hydrolysis will otherwiserapidly lose its activity for use in the present processes, as it beginsto polymerize. The resulting hydrolyzed organic silicon compound istherefore preferably mixed with the slurry of kaolin within 24 hours ofits hydrolysis, and more preferably within 14 hours of its hydrolysis.

The quantity of water added for the hydrolysis is preferably on about amole to mole basis with the silicon compound, or slightly less wheremoisture on the clay can make up the difference. Excess water ispreferably avoided, since it can prematurely hasten polymerization ofthe product with undesired precipitation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing appended hereto,

The FIGURE is a graph which compares the particle size distributions forthe starting feed clay and an aggregated product prepared by treatmentwith 2.2% equivalent tetraethoxysilane, as described in Example 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is demonstrated in the following Examples which areintended to be illustrative but not limiting.

Preparation of Feed Clay

The starting crude material for the below Examples was a blend of crudekaolins which were derived from northeast Georgia, and which werecomprised of very fine particle size materials. The GE brightness of thesaid crude was in the range of 82 to 87. In all instances in thisspecification it will be understood that brightness values are obtainedaccording to the standard specification established by TAPPI procedureT-646 os-75.

The crude clays were beneficiated according to the general practice usedin kaolinite processing industries. The crudes were thus first blungedand degritted, and thereupon were classified by centrifugation to 96%less than 1 micrometer E.S.D. and about 50-60%<0.25 micrometers E.S.D.(equivalent spherical diameter). The classified clays were flocculatedby using 0.25% by weight of aluminum sulfate and adjusting the pH to 3.5with sulfuric acid. The flocculated kaolinite was filtered. Thesignificantly dried (about 20% moisture remains) kaolinite wasredispersed with 0.25% by weight of sodium polyacrylate and the pHadjusted to about 7.0. 3% by weight of Carbital® 90, a ground calciumcarbonate having a P.S.D. of 90%<2 μm, was added to the slurry as anaggregation enhancing agent. The redispersed kaolinite was then spraydried.

The feed clay from the foregoing was provided to a Turbulizer® andreacted with an aerosol of a solution of aggregating agent solution asdescribed in the Examples.

The Turbulizer® (manufactured by Bepex Corporation of Minneapolis,Minn.), is a continuous mixing device including a horizontally orientedcylindrical chamber in which pitched blades are journaled about ahorizontal axis and are rotated at high speed. The kaolin suspended inair entered the chamber peripherally toward one end. Three spray nozzlesfed with the aggregation solution and compressed air dispersed theaggregating solution as an aerosol, the nozzles being mounted at spacedpoints along the wall of the Turbulizer® to inject the aerosol directlyto the interior of the chamber for mixing with the kaolin suspension.The treated feed clay emerging from the Turbulizer® was then taken asthe aggregated pigment product.

The relative sedimentation volume of treated and starting material wasmeasured to determine the extent and nature of aggregation. In theprocedure, a nearly 55% solids slurry of pigment was prepared thatcontained 0.3 milliliter of sodium polyacrylate. This slurry was thenspun at 7000 rpm for 45 minutes. The sediment volume was calculatedusing dry weight of clay, wet weight of clay, and calculating volume ofclay using density of dry clay of 2.6 g/ml.

Example 1

An aggregating agent solution was prepared by mixing 100 parts oftetraethoxysilane, 50 parts of ethyl alcohol, 17.5 parts of water and200 parts per million of sulfuric acid. This aggregating agent solutionwas atomized in an air stream and as such fed to the Turbulizer®, whichfrom a separate port was provided the aforementioned feed kaolin. Thequantity of the silane was such as to provide a dosage of 2.2% of thedry equivalent weight of the feed clay. The clay and treating solutionremained in the Turbulizer® mixing zone for 30 seconds to 1 minute. Theproduct was taken directly from the Turbulizer® and analyzed. The FIGUREdepicts the particle size distribution (P.S.D.) of the product and ofthe feed clay, showing that the fraction below 0.25 μm E.S.D. wasreduced from about 47% to about 37% as a result of treatment with theaggregating agent.

When this aggregated product is used as a paper filler or as a papercoating pigment, as in the formulations of Examples 1 and 10 of U.S.Pat. No. 4,935,062, improvements in the normalized light scatter (forthe filled sheets), and in such properties as sheet gloss, print glossand opacity (for the coated sheets) can be obtained, as compared to useof the unaggregated feed clay.

Example 2

An aggregating solution of tetraethoxysilane (TEOS) was used to treat afeed clay as in Example 1 above, except that 3.00% of the (TEOS)aggregating agent was added based on the dry equivalent weight of feedclay. The sub- 0.25 μm content was reduced from about 42% for the feedclay, to about 28% for the aggregated product.

While the present invention has been particularly set forth in terms ofspecific embodiments thereof, it will be understood in view of thepresent disclosure, that numerous variations upon the invention are nowenabled to those skilled in the art, which variations yet reside withinthe scope of the instant teachings. Accordingly, the invention is to bebroadly construed, and limited only by the scope and spirit of theclaims now appended hereto.

What is claimed is:
 1. A method for producing a chemically aggregatedkaolin pigment providing enhanced optical and printability propertieswhen used in paper manufacture, which comprises contacting asubstantially dry fine particle size feed kaolin with a solution of anaggregating agent comprising a previously hydrolyzed organic siliconcompound selected from the group consisting of symmetric compoundshaving the formula ##STR6## where R=CH₃, C₂ H₅, C₃ H₇, n--C₄ H₉ sec-C₄H₉ and C₆ H₅ and asymmetric compounds having the formula ##STR7## whereR=C₂ H₅, X=Cl or Br, and R'=H, and recovering an aggregated particulatekaolin pigment as product.
 2. A method in accordance with claim 1,wherein said solution is formed into an aerosol for said contacting withsaid kaolin.
 3. A method in accordance with claim 2, wherein saidaerosol is sprayed onto said kaolin.
 4. A method in accordance withclaim 2, wherein said solution and said kaolin are contacted with oneanother while being commonly dispersed in and mixed in a gas phase.
 5. Amethod in accordance with claim 4, wherein said kaolin and said solutionare mixed in a chamber provided with an agitator.
 6. A method inaccordance with claim 4, wherein said hydrolyzed silane is added as 0.5to 3% by dry weight of said kaolin.
 7. A method in accordance with claim6, in which the organic silicon compound is hydrolyzed by mixing saidcompound with water in approximately a molar ratio to said siliconcompound, together with a catalytically active quantity of an acid orbase catalyst for said hydrolysis.
 8. A method in accordance with claim7, further including mixing with said silicon compound and water, analcohol having solvating properties for both said water and said siliconcompound.
 9. A method in accordance with claim 8, in which said alcoholcomprises ethanol.
 10. A method in accordance with claim 8, in whichsaid alcohol comprises methanol.
 11. A method in accordance with claim8, in which the resulting hydrolyzed organic silicon compound iscontacted with said kaolin within 24 hours of its hydrolysis.
 12. Amethod in accordance with claim 8, in which the resulting hydrolyzedorganic silicon compound is contacted with said kaolin within 14 hoursof its hydrolysis.
 13. A method in accordance with claim 8, in whichsaid organic silicon compound comprises tetraethoxysilane.
 14. A methodin accordance with claim 8, in which said organic silicon compoundcomprises tetramethoxysilane.
 15. A method in accordance with claim 2,in which the kaolin feed particles are additionally mixed with anaggregation enhancing agent selected from the group consisting of thealkaline earth metal salts.
 16. A method in accordance with claim 15, inwhich said aggregation enhancing agent comprises calcium carbonate. 17.A method in accordance with claim 15, in which the aggregation-enhancingagent is present in the range of from 0.5% to 5% by weight of the kaolinon a dry basis.
 18. A method in accordance with claim 17, in which theaggregation-enhancing agent is present in the range of 2% to 4% byweight of the kaolin on a dry basis.
 19. A method in accordance withclaim 2, in which at least 98% by weight of the feed kaolin particlesare finer than 2 micrometers E.S.D.
 20. A method in accordance withclaim 1, wherein said hydrolyzed silane is added as 0.5 to 3% by weightof said kaolin.
 21. A method in accordance with claim 20, in which theorganic silicon compound is hydrolyzed by mixing said compound withwater in approximately a molar ratio to said silicon compound, togetherwith a catalytically active quantity of an acid or base catalyst forsaid hydrolysis.
 22. A method in accordance with claim 21, furtherincluding mixing with said silicon compound and water, an alcohol havingsolvating properties for both said water and said silicon compound. 23.A method in accordance with claim 22, in which said alcohol comprisesethanol.
 24. A method in accordance with claim 22, in which said alcoholcomprises methanol.
 25. A method in accordance with claim 22, in whichthe resulting hydrolyzed organic silicon compound is contacted with saidkaolin within 24 hours of its hydrolysis.
 26. A method in accordancewith claim 22, in which the resulting hydrolyzed organic siliconcompound is contacted with said kaolin within 14 hours of itshydrolysis.
 27. A method in accordance with claim 22, in which saidorganic silicon compound comprises tetraethoxysilane.
 28. A method inaccordance with claim 22, in which said organic silicon compoundcomprises tetramethoxysilane.
 29. A method in accordance with claim 1,in which the kaolin feed particles are additionally mixed with anaggregation enhancing agent selected from the group consisting of thealkaline earth metal salts.
 30. A method in accordance with claim 29, inwhich said aggregation enhancing agent comprises calcium carbonate. 31.A method in accordance with claim 29, in which the aggregation-enhancingagent is present in the range of from 0.5% to 5% by weight of the kaolinon a dry basis.
 32. A method in accordance with claim 1, in which atleast 98% by weight of the feed kaolin particles are finer than 2micrometers E.S.D.